Multi-Phase Thermal Control Apparatus, Evaporators and Methods of Manufacture Thereof

ABSTRACT

Multi-phase thermal control systems, evaporators, variable porous wick elements, heat transfer structures, and methods for their production are provided. Two-phase evaporators for use in such multi-phase thermal control systems are also provided. Two-phase evaporators incorporate a vapor plate body having there three major layers: a vapor channel network, a wick, and a liquid channel. The vapor channel network comprises a plurality of extrusions (e.g., vapor pillars) and associated channels (e.g., vapor channels) configured to allow a vapor to flow therethrough. The wick comprises a porous body configured to be disposed between the vapor channel network of and the liquid flow reservoir.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/666,217, filed Aug. 1, 2017, entitled “Multi-Phase Thermal ControlApparatus, Evaporators and Methods of Manufacture Thereof” to Roberts etal., which claims priority to U.S. Patent Application Ser. No.62/507,695, filed May 17, 2017 and U.S. Patent Application No.62/369,586, filed Aug. 1, 2016, the disclosures of which areincorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention described herein was made in the performance of work undera NASA contract NNN12AA01C, and is subject to the provisions of PublicLaw 96-517 (35 USC 202) in which the Contractor has elected to retaintitle.

FIELD OF THE INVENTION

The invention is generally directed to multi-phase thermal controlsystems, components thereof, and methods of their manufacture.

BACKGROUND

Multi-phase heat transfer can provide order of magnitude improvementsover single-phase systems in terms of system resource usage. Inaddition, it can be enabling for future instruments by lowering thethermal-related noise floor through its high degree of temperaturestability. Multi-phase heat transfer relies on the proper management ofliquid, vapor, and sometimes solid phases of the heat transfer fluid.The current state-of-the-art utilizes a porous media to provide thisphase management as well as capillary-based pumping for autonomousoperation. Conventional systems use sintered metals to provide suchmedia, albeit with a homogenous porosity. These parts are thenintegrated into heat acquisition and heat rejection devices.

SUMMARY OF THE INVENTION

In many embodiments, the invention is directed to multi-phase thermalcontrol systems, and methods of their manufacture.

Some embodiments are directed to a multi-phase evaporator including:

-   -   an evaporator body having at least one outer heating surface and        defining an internal volume;    -   a vapor channel network comprising a plurality of elongated heat        transfer elements separated by a plurality of channels, wherein        the elongated heat transfer elements have first ends        interconnected with an inner wall of the internal volume        opposite at least one outer heating surface such that the        elongated heat transfer elements and the outer heating surface        are in thermal contact, and having second ends distal from inner        wall;    -   a wick element comprising a liquid porous body disposed in        contact with the second ends of the plurality of elongated heat        transfer elements;    -   a fluid reservoir disposed within the internal volume such that        the wick element is interposed between the fluid reservoir and        the vapor channel network;    -   at least one fluid inlet disposed in a wall of the evaporator        body such that fluid flows into the fluid reservoir; and    -   at least one outlet disposed in a wall of the evaporator body        distal from the at least one fluid inlet, the at least one        outlet configured to transport both heated vapor and heated        liquid from the evaporator body.

In other embodiments the multi-phase evaporator includes:

-   -   at least two outlets, wherein at least a first outlet is        configured to transport heated liquid from the fluid reservoir,        and wherein at least a second outlet is configured to transport        heated vapor from the vapor channel network; and    -   wherein the wick element is disposed within the evaporator body        and configured as a phase separator between the vapor channel        network and the fluid reservoir.

In still other embodiments the wick element spans the entire openingbetween the fluid reservoir and the vapor channel network.

In yet other embodiments the multi-phase evaporator includes at leastone mixed-phase outlet configured to transport both heated vapor andheated liquid from the evaporator, and wherein the wick element onlypartially spans the opening between the fluid reservoir and the vaporchannel network such that a mixing region is formed adjacent the atleast one mixed-phase outlet where heated vapor from the vapor channelnetwork and heated liquid from the fluid reservoir interact prior toflowing into the at least one mixed-phase outlet.

In still yet other embodiments the multi-phase evaporator includes anevaporator cap configured to seal the internal volume adjacent the fluidreservoir.

In still yet other embodiments the evaporator cap further comprises aplurality of depressions formed into the inner surface thereof, theplurality of depressions being configured to engage a plurality ofresilient members configured to engage and apply a preload force ontothe wick element such that thermal contact is maintained between thewick element and the second surfaces of the plurality of elongated heattransfer elements.

In still yet other embodiments the wick element is formed of a porousmaterial having variable porosity.

In still yet other embodiments the porosity of the wick element variescontinuously through the cross-section of the wick element.

In still yet other embodiments the wick element is integrally formedinto the body of the evaporator.

In still yet other embodiments the wick element serves as a structuralmember of the body of the evaporator.

In still yet other embodiments the at least one heating surface isconfigured to conform with the body of an external heat generatingelement.

In still yet other embodiments the multi-phase evaporator includes aplurality of resilient members configured to engage and apply a preloadforce onto the wick element such that thermal contact is maintainedbetween the wick element and the second surfaces of the plurality ofelongated heat transfer elements.

In still yet other embodiments the plurality of elongated heat transferelements comprise pillars having a uniform geometric cross-section, thepillars being one of either solid or porous.

In still yet other embodiments the ratio of the width of a lengthdetermined by the width of an elongated heat transfer element andadjacent vapor channel to the width of the elongated heat transferelement is from 0.5 to 0.8.

In still yet other embodiments the width of the elongated heat transferelement is from 0.25 to 0.4 inches.

In still yet other embodiments the walls of the evaporator are formedfrom low thermally conducting materials.

Still other embodiments are directed to a thermal control systemincluding:

-   -   a fluid handling system comprising a plurality of interconnected        fluid conduits;    -   a pump in fluid communication with the fluid handling system;    -   an evaporator in fluid communication with the fluid handling        system and configured to pick up a heat load comprising:    -   an evaporator body having at least one outer heating surface an        defining an internal volume,        -   a vapor channel network comprising a plurality of elongated            heat transfer elements separated by a plurality of channels,            wherein the elongated heat transfer elements have first ends            interconnected with an inner wall of the internal volume            opposite the at least one outer heating surface such that            the elongated heat transfer elements and the outer heating            surface are in thermal contact, and having second ends            distal from inner wall,        -   a wick element comprising a liquid porous body disposed in            contact with the second ends of the plurality of elongated            heat transfer elements,        -   a fluid reservoir disposed within the internal volume such            that the wick element is interposed between the fluid            reservoir and the vapor channel network,        -   at least one fluid inlet disposed in a wall of the            evaporator body such that fluid flows into the fluid            reservoir, and        -   at least one outlet disposed in a wall of the evaporator            body distal from the at least one fluid inlet, the at least            one outlet configured to transport both heated vapor and            heated liquid from the evaporator body;    -   a condenser in fluid communication with the fluid handling        system downstream of the evaporator and configured to reject the        heat load; and    -   an accumulator in fluid communication with the fluid handling        system disposed at an inlet of the pump and configured to        regulate the system pressure.

In other embodiments the thermal control system includes:

-   -   at least two outlets, wherein at least a first outlet is        configured to transport heated liquid from the fluid reservoir,        and wherein at least a second outlet is configured to transport        heated vapor from the vapor channel network; and    -   wherein the wick element is disposed within the evaporator body        and configured as a phase separator between the vapor channel        network and the    -   fluid reservoir.

In still other embodiments the thermal control system includes at leastone mixed-phase outlet configured to transport both heated vapor andheated liquid from the evaporator, and wherein the wick element onlypartially spans the opening between the fluid reservoir and the vaporchannel network such that a mixing region is formed adjacent the atleast one mixed-phase outlet where heated vapor from the vapor channelnetwork and heated liquid from the fluid reservoir interact prior toflowing into the at least one mixed-phase outlet.

In yet other embodiments the thermal control system of claim 19, furthercomprising a pre-heater in fluid communication with the fluid handlingsystem upstream of the evaporator.

In still yet other embodiments the wick element is formed of a porousmaterial having variable porosity.

In still yet other embodiments the thermal control system includes aplurality of resilient members configured to engage and apply a preloadforce onto the wick element such that thermal contact is maintainedbetween the wick element and the second surfaces of the plurality ofelongated heat transfer elements.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description and claims will be more fully understood with referenceto the following figures and data graphs, which are presented asexemplary embodiments of the invention and should not be construed as acomplete recitation of the scope of the invention.

FIG. 1 provides a schematic on a multi-phase evaporator according toembodiments.

FIG. 2a provides a schematic of an evaporator body of a multi-phaseevaporator according to embodiments.

FIG. 2b provides a schematic of an evaporator cap of multi-phaseevaporator according to embodiments.

FIG. 3 provides a schematic depiction of a unit cell for vapor networkdesign according to embodiments.

FIG. 4 provides a data plot of the fraction of the heated surface incontact with the wick as a function of the pillar length to cell lengthratio according to embodiments.

FIG. 5 provides a data plot of the vapor buildup factor, which is anormalized pressure drop of vapor flow through the wick, as shown in EQ.4 according to embodiments.

FIG. 6 provides a data plot of the pressure drop as a function of thevapor channel width, or Lc-Lp according to embodiments.

FIG. 7 provides a data plot of the critical heat flux of a variableporosity wick element according to conventional technology.

FIG. 8a provides images of 3-D printed porous wick elements according toembodiments at time zero (top) and a few seconds later (bottom) aftercontact with an isopropanol liquid.

FIG. 8b provides an image of a multi-porous wick element structure madefrom 3-D printed aluminum according to embodiments, where liquid wickingrates are shown to vary, which is indicative of successful bi-porosityon a monolithic part.

FIG. 9 provides a schematic of a basic Separated Flow Architecture (SFA)system (left) and a Mixed Flow Architecture (MFA) system (right), wherethe fluid flows in the direction of the ascending numbers according toembodiments.

FIGS. 10a and 10b provide schematics of a SFA evaporator (10 a) and MFAevaporator (10 b) according to embodiments.

FIGS. 11a and 11b provide section views of the evaporator design for theSFA (11 a) and SFA testbeds (11 b) according to embodiments.

FIG. 12 provides a schematic of the operation of a MFA evaporatoraccording to embodiments.

FIG. 13 provides a schematic of a mechanically pumped two-phase coolingloop with system operating conditions according to embodiments.

FIG. 14 provides a data plot of the expected pressure drops in piping ofentire cooling loop are shown for two different flow rates and pipeinner diameters (assumed dimensions are 12 feet of piping, with 25%two-phase flow for the section between the evaporator and condenser, and75% liquid flow in the remaining piping sections) according toembodiments.

FIG. 15a provides a data plot taken from rate of rise test for stainlesssteel wick (where slope and intercept are used to determine permeabilityand pore size) according to embodiments.

FIG. 15b provides a data plot taken from rate of rise test for wool feltwick (where slope and intercept are used to determine permeability andpore size) according to embodiments.

FIGS. 15c and 15d provide data plots of linear fit calibration dataaccording to embodiments.

FIG. 16 provides a data plot of pressure vs. position for an MFA systemaccording to embodiments.

FIG. 17 provides a schematic of an MFA testbed according to embodiments.

FIGS. 18a and 18b provide plots of experimental data for the MFA systemset to produce an isothermal evaporator with heat loads ranging from 200W to 390 W according to embodiments.

FIG. 19 provides IR images of the MFA evaporator with three differentheat loads: 200 W, 315 W, and 390 W according to embodiments.

FIG. 20 provides an IR image of the evaporator with a small heater usedto study the effect of subcooling on the isothermality of theevaporator.

FIG. 21 provides a schematic of the MFA system pressures and flowresistances according to embodiments.

FIG. 22 provides a data plot of pressure vs. position for an SFA systemaccording to embodiments.

FIG. 23 provides a schematic of the section of the SFA system that isbeing modelled (above), and the corresponding circuit diagram (below)according to embodiments.

FIG. 24 provides a schematic of an SFA testbed according to embodiments.

FIG. 25 provides plots of experimental data for the SFA system accordingto embodiments.

FIG. 26 provides data plots of the SFA system undergoing stabilitytesting according to embodiments.

FIG. 27 provides an IR image of steady state evaporator according toembodiments.

DETAILED DESCRIPTION

Turning now to the drawings and data, multi-phase thermal controlsystems, variable porous wick heat transfer structures, and methods fortheir production are provided. Many embodiments are directed totwo-phase evaporators for use in such multi-phase thermal controlsystems. In various embodiments, such two-phase evaporators incorporatea vapor plate body having there three major layers: a vapor channelnetwork, a wick element, and a liquid channel. In many such embodimentsthe vapor channel network comprises a plurality of extrusions (e.g.,vapor pillars) and associated channels (e.g., vapor channels) configuredto allow a vapor to flow therethrough. In various embodiments, the wickelement comprises a porous body configured to be disposed between thevapor channel network of and the liquid flow reservoir. In manyembodiments, the wick element has a variable porosity. In some suchembodiments, the wick element may be made using a stochastic additivemanufacturing process such that the wick element may take anyconfiguration and/or such that the wick element may be directlyintegrated into the body of the evaporator as a unitary piece thereof.

Although certain embodiments will be described in the followingdiscussion, it will be understood that the nature of the vapor channelnetwork, the wick element and the liquid reservoir may be modified asnecessary to suit required heat flow conditions, such conditionsdeterminable through methods further described herein.

In various embodiments, the multi-phase evaporator in accordance withembodiments is disposed within a larger multi-phase heat transfer systemin association with suitable fluid transfer conduits, pumps, filters,condensors etc. as will be readily configurable by those skilled in theart. During operation of embodiments of such heat transfer systems, oneor more two-phase evaporators are disposed in such a fluid handlingsystem and the outer face of the vapor network is placed into thermalcontact with a surface the temperature of which is to be managed. Inmany embodiments, the evaporator is configured such that as fluid fillsthe reservoir, the wick becomes saturated with the fluid, and heat isconduct through the extrusions in the vapor channel network to heat thewick and evaporate the fluid at the surface of the wick adjacent to thevapor channels such that the vapor is released into the vapor channelnetwork. This vapor then spreads and percolates through the network ofchannels, spreading away from the heated area. Embodiments of suchmulti-phase heat transfer systems may incorporate mixed-phase andsingle-phase architectures. The operation of such devices according toembodiments will be described in greater detail below.

Thermal management is becoming more and more critical, particularly aselectronics are miniaturized and the need for the heat flux increases.The current state-of-the-art for heat transfer is a mechanically pumpedsingle-phase cooling loop. In this system, the working fluid rejectsheat by raising its bulk temperature. (See, e.g., Zhang, H., et al.,Applied Thermal Engineering 25, 1472-1487 (2005), the disclosure ofwhich is incorporated herein by reference.) While this design provides arobust system, it requires a substantial amount of mass and pumpingpower, and provides low effectiveness in rejecting heat. Additionally,there are capillary pumped two-phase cooling loops, where a wickstructure produces a pressure gradient for a working fluid to flowthrough the system while vaporizing and condensing to reject heat. (See,e.g., Liu, Z., et al., Journal of Thermophysics and Heat Transfer 22,98-104 (2008), the disclosure of which is incorporated herein byreference.) These systems take advantage of the high heat ofvaporization by way of phase change and do not require an external pump,but do not offer much geometric flexibility for heat source location andare sensitive to varying gravitational conditions. (See, e.g., Park, C.,Vallury, A. & Zuo, J. Journal of Thermal Science and EngineeringApplications 1, 022004 (2009), the disclosure of which is incorporatedherein by reference.) A new technology that combines the best of thesetwo systems is a mechanically pumped two-phase cooling loop. Using phasechange, such systems are able to reduce system mass substantially withmore efficient heat transfer, while the mechanical pumping provides thecapability to reach distributed heat loads while controlling thetemperature to produce an isothermal surface.

In such mechanically pumped two-phase cooling loops, the evaporatorcomponent of the mechanically pumped two-phase loop is critical towardsoperation. This is partly because in poorly designed evaporators vaporbubbles can grow large and block the heated surface from direct liquidcontact, thereby substantially increasing thermal resistance. Unless theflow rate is excessively high to operate in inertia-dominated flowconditions, traditional evaporators experience premature dry out andtherefore degraded heat transfer. (Konishi, C. & Mudawar, I.International Journal of Heat and Mass Transfer 80, 469-493 (2015), thedisclosure of which is incorporated herein by reference.)

Embodiments of Multi-Phase Heat Transfer Systems

Many embodiments are directed to a multi-phase evaporator generallycomprising a vapor region and a liquid region separated by porous wick.In various embodiments, the multi-phase evaporator comprises threegeneral constructs: a vapor plate having a vapor channel network, aliquid flow reservoir, and a porous wick material disposed therebetween.Such an evaporator architecture forms an isothermal heat acquisitionsurface since during nominal operation, only vapor (close to thesaturation temperature) is present in the vapor channel, and as aresult, any subcooled liquid is kept away from the heated surface by thewick.

An exemplary embodiment of the multi-phase evaporator is depicted inFIG. 1. As shown, the multi-phase evaporator generally comprises anevaporator body (10) defining an internal volume (12) and at least vapor(14) and liquid (16) inlets. Within the internal volume is disposed avapor channel network (18) defined by a plurality of elongated heattransfer features and channels through which the vapor will flow. Aporous wick element (20) is disposed in thermal transfer arrangement ontop of the heat transfer features of the vapor channel network. Anevaporator cap (22) is then disposed atop the evaporator body (10) suchthat a fluid reservoir is formed between the wick and the evaporatorcap. In accordance with embodiments, the evaporator may be a unitarybody or a multi-element body. Sealing bolts (24) and an 0-ring channel(26) may be provided to form a sealed system in embodiments where amulti-element body is used.

A detailed view of the primary elements of a multi-phase evaporator inaccordance with embodiments is provided in FIGS. 2a and 2 b. As shown inFIG. 2 a, the evaporator body (30) comprises an inner volume (32) havinga vapor channel network (34) formed of a plurality of elongated heattransfer features (36) separated by a network of vapor channels (38).Although heat transfer features formed of elongated solid body squarepillars are shown in the figures, it will be understood that the heattransfer features may take a number of alternative forms including, forexample, different geometric cross-sections (e.g., round, oval,hexagonal, etc.), and may be porous or solid. A fluid inlet (40) isdisposed at a height on the evaporator body such that it is above thelevel on which the wick element (not shown) is positioned (e.g., atopthe heat transfer features (36) of the vapor channel network) such thatcooling liquid flows into the fluid reservoir (42) and is separated fromthe vapor channel network by the porous body of the wick element. In theembodiment shown in FIG. 2 a, a separate edging (44) is provided toconstrain movement of the wick element and ensure a fluid tight sealbetween the vapor channel network (34) and the fluid reservoir (42),which is defined by the portion of the inner volume (32) of theevaporator body between the wick element and the enclosing cap (notshown). At least one fluid outlet (45) is also provided such that heatedfluid and/or vapor may be discharged from the evaporator. As will bedescribed in greater detail below, in accordance with embodiments, thenumber and positioning of the wick element and the outlet will depend onwhether the evaporator is to operate in a single or mixed-phaseconfiguration. As shown, in multi-element evaporator bodies, (i.e., notunitary constructions) the evaporator body may be provided with achannel (46) for containing an o-ring seal, and cooperating bolt holes(48) to allow the evaporator cap to be secured thereto such that afluid-tight seal is formed therebetween.

Because the evaporating environment needs to be at saturated conditionswith no foreign gases, a secure hermetic seal needs to be made forproper operation. In embodiments, this can be accomplished by using aunitary body, welding the two plates, alternatively welding of inlet andoutlet plenums on either side of the evaporator, or the two plates maybe designed to be sealed by an 0-ring using bolts around the outside toclamp the plates together, as shown in FIGS. 2a and 2 b.

An exemplary embodiment of the cap of a sealed system is provided inFIG. 2 b. As shown, the evaporator cap (50) can, according to someembodiments, contain cooperative bolt holes (52) and can, in someembodiments, be provided with a plurality of depressions (54) disposedon the inner surface thereof. These depressions can be formed to providesecuring points into which springs or other resilient elements may beplaced to provide a preload force on the wick element to ensure thermalcontact is maintained between the wick element and the thermal transferfeatures of the vapor channel network.

Although a specific arrangement of features, including channel and heattransfer features, inlets and outlets, wick element position, resilientmember, evaporator cap, and resilient elements have been described, itwill be understood that the number and arrangement of these elements maytake many alternative configurations in accordance with embodiments, aswill be described in greater detail below.

Liquid Channel Design

There are two general concerns for the liquid channel: to avoid a largepressure drop and to have a large enough volume of cooling liquid suchthat the wick element can continuously have a liquid supply for wettingthe heated surface. In this case, both of these requirements tend towardembodiments having a larger liquid channel. Accordingly, in manyembodiments, as shown in FIGS. 1 and 2, a single, voluminous reservoirthrough which the liquid flows is provided. As a liquid travelingthrough such a larger reservoir will travel at a lower velocity, thepressure drop, which can be calculated using techniques, which aredescribed in greater detail in the Exemplary Embodiments, is negligible.Therefore, the main consideration is having a channel, which cancontinuously provide liquid to saturate the wick element. For example,with a wick of 0.25″ that is generally around 35% empty space, a heightof liquid equivalent to 2.5 times the amount of liquid in a given columnof the wick element may be chosen, which works out to be 0.225″. Thisensures that even in areas near the discrete heat loads, there will beplenty of liquid available to re-wet the wick element and have liquidcoverage on the boiling surface. Details on making these calculationsare also provided in greater detail below.

Heated Surface and Vapor Channel Design

As shown in the figures, the vapor channel network is configured as atwo-dimensional network of channels such that vapor is not concentratein a particular area where a discrete heat source is placed. By allowingthe vapor to spread out across the area of the plate, the vapor not onlyhas a larger effective hydraulic diameter through which to flow and thusreduce pressure drop, but it will also be less likely to expand backinto the wick and displace liquid from the heated surface. The processof determining the sizing, spacing, and frequency of these channels isnot trivial. Following are the three major considerations to optimizethe heat transfer from the heat transfer features to the wick element aswell as minimize the pressure drop of the higher velocity vapor flowthrough the channels. To quantify the vapor channel design, consider aunit cell of the vapor network shown in FIG. 3, with importantparameters L_(p) being the dimension of the pillar and L_(c) being thedimension of the unit cell.

Wick Contact Area with Heated Surface: The first consideration is thecontact area of the wick element with the heated surface of theevaporator body. As the body in accordance with embodiments is made fromhighly conductive material, such as, for example, aluminum or copper,the heat will spread and provide a nearly uniform local heat flux to theheated surface. Therefore, to maximize the heat transfer to the wickelement where the liquid evaporates, in accordance with embodiments, thelargest possible percentage of the heated surface is to be in contactwith the wick element, or the ratio L_(p)/L_(c) is to approach 1. Thus,the heat transfer contact area goes as (L_(p)/L_(c))², as plotted inFIG. 4. Accordingly, in many embodiments for high heat transfer, a highratio of Lp=Lc is desired.

Vapor Buildup in Wick: The next consideration is avoiding vapor buildupin the wick element at the heat transfer element surfaces. While largeheat transfer elements ensure that more of the heat source getsdelivered to the wick element surface, they also prohibit vapor fromescaping into the vapor channels. In the limit where L_(p)=L_(c), theliquid would vaporize at the surface of the wick element, have no vaporchannel to enter, and simply displace liquid out of the wick elementjust like in normal film boiling. In the limit where L_(c)→0, vaporforms at the surface of the wick element and has no distance to travelbefore releasing into the channel and allowing the wick to re-wet. Areasonable representation to quantify this phenomenon is by use ofDarcy's Law for flow in porous media, given by EQ. 1.

$\begin{matrix}{Q = {{- \frac{KA}{\mu_{1}}}\frac{\Delta P}{\Delta x}}} & \left( {{EQ}.\mspace{11mu} 1} \right)\end{matrix}$

Since a smaller pressure drop of vapor in the wick element correspondsto a preferred design, rearranging for ΔP will present a useful form ofEQ. 1. A priori, the relevant length scales for flow rate Q=εL² _(p)h/Δtand cross sectional area of flow towards the vapor channel A=L_(p)h areknown, where e is the wick porosity, h is the height of the vapor layer,and Δt is the time scale of flow. However, the length Δx in which thevapor flows through the wick element is a function of the heat transferelement size, and must be computed with an integral. EQ. 2 computes theaverage length Δx_(avg) over the extent of the vapor channel networkarea that the vapor travels for a given pillar size.

$\begin{matrix}{{\Delta x_{avg}} = {\frac{\int{\Delta xdA}}{A} = {\frac{\int_{0}^{L{p/2}}{\left( {\frac{L}{2} - x} \right)8xdx}}{L_{p}^{2}} = \frac{Lp}{6}}}} & \left( {{EQ}.\mspace{11mu} 2} \right)\end{matrix}$

Therefore, in total the expression for pressure drop is given by:

$\begin{matrix}{{\Delta P} = {\frac{\mu_{l}}{6K\Delta t}L_{p}^{2}}} & \left( {{EQ}.\mspace{11mu} 3} \right)\end{matrix}$

Since this formulation is an approximation, it may be misleading tocompare absolute pressure drops. Therefore, EQ. 4 defines the vaporbuildup factor according to:

$\begin{matrix}{f_{vb} = {{- \frac{6K\Delta P\Delta t}{\mu_{l}L_{norm}^{2}}} = \frac{L_{p}^{2}}{L_{norm}^{2}}}} & \left( {{EQ}.\mspace{11mu} 4} \right)\end{matrix}$

The vapor buildup factor is a normalized pressure drop to isolate thedependence on the vapor pillar length and easily compare betweendifferent sized heat transfer elements. In summary, the vapor buildupfactor in accordance with embodiments is plotted in FIG. 5, displayingan L_(p) ² dependence. Accordingly, in many embodiments a low value ofL_(p) is desired as it results in the vapor having less flow resistanceto escape into the channels, thereby limiting vapor buildup in the wickelement.

Pressure Drop in Vapor Channels: The final consideration is the pressuredrop through the vapor channels, given by EQ. 5. Because the density ofvapor is so much lower than that of the liquid, the vapor pressure dropcan become significant due to a strong dependence on hydraulic diameter.This outlines another inherent trade-off between maximizing the area ofcontact with the wick element for high heat transfer versus minimizingthe pressure drop with larger sized channels. That is, as L_(p)→L_(c),the pressure drop approaches infinity due to the D⁻⁴ _(hyd) dependencein EQ. 5. In short, the pressure drop through the channels goesapproximately as (L_(c-)L_(p))⁻⁴ as shown in FIG. 6. Accordingly, inmany embodiments, excessively small channels are avoided to provide areasonable pressure drop.

$\begin{matrix}{{\Delta P_{vap}} = {12S\frac{\mu_{v}L_{eff}V}{\pi D_{hyd}^{4}}}} & \left( {{EQ}.\mspace{11mu} 5} \right)\end{matrix}$

Accordingly, in designing the heat transfer network of the evaporator inaccordance with embodiments, one should go from most to least sensitiveto determine acceptable operating conditions. First, looking at FIG. 6showing the absolute pressure drop values, a sharp increase occurs priorto 0.100″. A large pressure drop would compensate the isothermality ofthe system due to the strong dependence of saturation temperature onsaturation pressure. In addition, even with the ability of the pump toaccount for these losses, avoiding such a steep slope by an incrementalincrease in channel width would be worth the pumping power saved.Therefore, in various embodiments, the vapor channel width (L_(c)-L_(p))is recommended to be from 0.100″ to 0.200″, and in some embodimentsaround 0.125″ for ease of manufacturing.

With this value chosen, in many embodiments, it is prudent to considerFIG. 5. The slope of the vapor buildup factor begins to increase moresubstantially at around L_(p)=0.300″. With the value of (L_(c)-L_(p))taken to be 0.125″ as before, this would correspond to a value ofL_(c)/L_(p) of from 0.5 to 0.8, and in some embodiments around 0.7, andthe wick element area coverage as seen in FIG. 4 would be around 50%.This is not ideal for area coverage, but the incremental increase in thevapor buildup factor is estimated to be equivalent to the incrementaldecrease in the wick area coverage due to the similar dependence onL_(p). Therefore, to submit to the tradeoff of these two effects, inembodiments, a heat transfer element length L_(p) in accordance withembodiments is recommended to be from around 0.250″ to 0.400″, and insome embodiments around 0.310″ in order be compatible with the wicksize.

Thermal Contact of Wick with Vapor Pillars

Another consideration for the evaporator according to embodiments is tohave a low thermal contact resistance between the wick element and theheat transfer elements. Without sufficient conductance, the path ofleast resistance would be through the vapor and negate the effect ofhaving a wick element provide liquid to the heated surface. Therefore,in embodiments a few measures may be employed in order to promote strongthermal contact.

First, although the heat transfer elements are square in shape in thefigures any suitable shape may be used, such as for example, circular,oval, octagonal, etc. Regardless of the shape, the edges of heattransfer elements could be protruding and prevent contact with themajority of the square face. To remedy this, in many embodiments an edgebreak less than 0.010″ in the vapor channel network may be used, whichtakes some material off the edges of the squares to uncover the largeflat surface of each heat transfer elements. Additionally, as describedabove, a series of depressions may be formed throughout the face of theevaporator cap to provide a small preload force over the area of thewick element by resilient elements (e.g., compression springs\), whichwill press the surface of the wick into contact with the heat transferelements. In many embodiments, the wick element may be made compliantenough to bend slightly and more cleanly contact with the heat transferelements. In alternative embodiments, a thermal interface material maybe interposed between the wick element and the heat transfer elements tofurther enhance the thermal contact.

Pressure Vessel Considerations

Because there will be a pressure differential between the ambientconditions and the evaporator chamber, the evaporator must bestructurally sound to avoid failure. Yet, the goal is to have the heatedsurface be as thin as possible to reduce the thermal mass for a moreefficient design. To ensure structural safety, simulations in Solidworkswere performed, applying a pressure force corresponding to 15 PSI on theinner chamber. In many embodiments, a thickness of 0.225″ was chosen forthe heated surface, which provides a safety factor of 2.5.

Variable Porosity Wick Elements and Methods of Manufacture

As previously discussed, multi-phase heat, transfer systems andevaporators can provide order of magnitude improvements oversingle-phase systems in terms of system resource usage. In addition, itcan be enabling for future instruments by lowering the thermal-relatednoise floor through its high degree of temperature stability. However,the thermal management technologies rely on the proper management ofliquid, vapor, and sometimes-solid phases of the heat transfer fluidthrough a porous wick element. Conventional wick elements use sinteredmetals having homogenous porosity. As shown in the figures discussedthus far, these wick elements are made separately and integrated intoheat acquisition and heat rejection devices, such as evaporators.

The limitations associated with such conventional sintered, homogenouswick elements include that the wick element needs to be manufacturedseparately from the rest of the heat transfer device, which results inlimitations to the device geometry. In addition, being a separateelement, additional thermal interfaces are introduced which havesignificant influence on the proper operation of the heat transferdevice. Thermal contact resistances and fluid leak paths are alsointroduced using such separate elements. Finally, sintered metal wickelements cannot precisely and accurately control the variability of theporosity through the media. As a result, sintered wick elements aregenerally homogenous, monolithic media, which limits custom devicedesign. Moreover, the gains achieved through enhanced wetting andcapillary pressure generation are limited by the relatively highpressure drops associated with flow through low porosity media.

Accordingly, many embodiments incorporate variable porous wick elements.The benefit to the critical heat flux limit associated with variableporous wick elements is depicted in FIG. 7. In various such embodiments,advanced manufacturing techniques, such as, for example, additivemanufacturing techniques may be used to form wick elements havingmonolithic and variable porosity structures. In some such embodiments,electron beam and laser sintering associated with metal 3-D printingtechniques may be used to produce porous metal (e.g., titanium,aluminum, etc.) structures having excellent wicking capability.Exemplary wick elements are shown in FIG. 8a (which shows a 3-D printedporous structure made from titanium) and FIG. 8b (which shows amultiporisity structure formed from aluminum).

Embodiments utilizing such additively manufactured, variable porous wickelements can enable new classed of heat transfer devices. In suchembodiments, the wick element structure need not be limited to anincremental set of porosities but rather can be continuously variable.In many such embodiments, porosity gradients may be utilized forimproving the performance of traditional evaporators as well as allowingfor the creation of evaporator structures of any conformation. Forexample, unlike conventional evaporator structure, which are constrainedby the wick element to be planar, the wick element, and, in turn, theentire evaporator can be configured to conform precisely to thecomponents from which heat needs to be transferred. In addition, becausethe wick element can be directly formed and integrated with the rest ofthe evaporator such that it can be directly integrated with thesupporting components, including, but not limited to, the heat transferelements, evaporator walls, inlets and outlets, etc. Accordingly, insuch embodiments supportive elements such as the resilient members canbe eliminated allowing for volume and mass savings.

Mixed-Phase & Single-Phase Configurations

Although the above discussion has focused on the design of themulti-phase evaporator and its various components, it should beunderstood that embodiments are also directed to large thermal transfersystems incorporating such evaporators. Specifically, embodiments ofevaporators may be incorporated into two different mechanically pumpedmulti-phase thermal control systems: separated flow systems or mixedflow systems. In mixed flow systems (MFA), two-phase flow exits theevaporator with the thermodynamic quality varying with heat load. Inseparated flow systems (SFA) the liquid and vapor phases are keptseparated in dedicated liquid and vapor lines. A schematic of these twoarchitectures in their simplest form is shown in FIG. 9. As shown, inmany embodiments both architectures contain the same basic elements: apump (to circulate the working fluid), an evaporator (to pick up theheat load), a condenser (to reject the heat load), and an accumulator(to regulate system pressure).

In conventional thermal transfer systems, the primary difference betweenthe two architectures lies in the design of the evaporator. In the SFA,the evaporator additionally functions as a phase separator and has twooutlets: one for vapor and one for liquid. In such configurations, thereis a liquid chamber and vapor chamber that are separated by a porouswick element, which maintains the liquid-vapor interface. The evaporatoracquires heat through the vapor chamber side by evaporating fluid at theliquid-vapor interface. In conventional MFA designs, the vapor andliquid phases are not separated in the evaporator and consequentlytwo-phase flow occurs both in the evaporator and in the flow linebetween the evaporator and condenser. In its simplest form the MFAutilizes flow boiling in the evaporator. FIGS. 10a and 10b schematicallyillustrate basic forms of the SFA (10 a) and MFA (10 b) evaporators. Thedifference in evaporator design between the two architectures typicallyleads to significant operational differences.

According to embodiments of thermal control systems according to thedisclosure a single evaporator design is used where variations in theoutlet configurations allow for operation of the evaporator in either aSFA or MFA architecture. FIGS. 11a and 11b provide cross-sectionalschematics of the evaporator in both configurations that illustrate theprinciple of operation. As shown, in embodiments, both evaporatorconfigurations (60) consist of a single chamber (62) that is divided bya porous wick element (64). The wick element is pressed against heattransfer element (66) which are connected to the heated surface (68).These heat transfer elements transfer heat to the wick element and causeevaporation of the working fluid at the pillar/wick interface. Duringoperation, vapor is formed on the heat transfer element side of the wickelement and flows downstream toward the evaporator outlet. On the otherside of the wick, liquid flows from the evaporator inlet (70) to theoutlet (72) in a parallel path. As shown in FIG. 11 a, in the SFA, thewick element (64) extends across the entire length of the fluidreservoir within the evaporator body such that the liquid and vaporstreams remains separated throughout the entire evaporator. As shown inFIG. 11 b, in the MFA, liquid and vapor streams are separated by theporous wick element until they recombine near the outlet (72) of theevaporator.

Apart from the differential configuration of the liquid and vaporstreams at the outlet to the evaporator, in both embodiments the wickelement serves two primary purposes: first it ensures that the heatedsurface remains wetted in a microgravity environment (to avoid dry out),and second it helps keep the heated surface isothermal by constrainingthe vapor flow to be next to the heated surface. For the SFA design, ithas the additional function of serving as a phase separator.

Accordingly, embodiments of the multi-phase evaporator may be utilizedin either SFA or MFA thermal transfer system architectures with a minormodification of the evaporator configuration. As shown schematically inFIG. 12, in both configurations cooling liquid comes through the centralinlet port on the left, spreads, and fills the cooling liquid reservoirchannel. As the liquid fills the channel, the porous wick element takesin liquid over its entire area until it saturates. As the heat source(s)is being applied to the heat transfer elements, the heat is conductedthrough the heat transfer elements to the wick element. As the wickelement is heated the cooling fluid is evaporated at the surface of thewick element adjacent to the vapor channels, and the vapor is releasedinto the vapor channel network. This vapor then spreads and percolatethrough the network of channels, spreading away from the heated area andavoiding superheating. Meanwhile, the wick element helps re-wet theevaporated area through its liquid spreading power. The only differencebetween the evaporator configurations is what happens to the liquid andvapor in exiting the evaporator. In an MFA system (shown) the vaporrejoins the excess liquid and exits the evaporator on the right, whereasin an SFA system (not shown) the vapor and liquid would remain separatedand exit through separate outlets.

EXEMPLARY EMBODIMENTS

Experiments were conducted to demonstrate the capabilities of theevaporators and thermal control system in accordance with embodiments.These results and discussion are not meant to be limiting, but merely toprovide examples of operative devices and their features.

EXAMPLE 1 Thermal Control System Configurations

Many exemplary embodiments are directed to determining configurationsfor the evaporator. In making such configuration determinations, theoperating conditions must be determined. In these exemplary embodiments,the working fluid will be water. Since the focus of the evaporator isthe performance of the boiling heat transfer, the system is also assumedto operate in saturated conditions. Lowering the system pressure to beless than atmospheric also allows for a lower saturation temperature andtherefore will minimize the amount of heat loss interaction with thesurrounding ambient conditions. To avoid pump cavitation, the systemneeds to be at a reasonable absolute pressure, varying depending on thepump specifications. With these considerations in mind, the operatingtemperature will be assumed to be approximately 60° C., whichcorresponds to a vapor pressure of 2.9 psia. FIG. 13 provides a summaryof the experimental setup.

Heat Loads and Flow Rate: To determine the expected pressure drop acrossthe evaporator, it is helpful to know the flow rate. In a two-phasesystem, the flow rate is directly linked to the heat load. For thespacecraft payloads being considered, the low heat fluxes of around 5W/cm² correspond to heat loads of around 500-1000 W total. Because ofthe selection of water as a working fluid, the high heat of vaporizationresults in extremely low flow rates on the order of 15-30 mL/minrequired to remove all of the heat from the heat sources. For practicalpurposes of keeping the wick wetted, the testing will operate at anexcess flow rate, meaning not all liquid will be converted to vapor.Therefore, the design range for flow rate is chosen to be 40-80 mL/min.

Pressure Drops in Piping: As a point of reference for designing theallowable pressure drop across the evaporator, it is worthwhile knowingapproximate values of the pressure drop through the piping of thecooling loop. The following subsections describe the methodology used todetermine the pressure drop of each tubing section, added together toyield FIG. 14. Because of the low flow rate, the pressure drops in thepiping are generally small and acceptable, which allows for designflexibility with regards to the evaporator features.

Pressure Drop in Liquid Section: Assuming laminar flow, the normalfriction factor formulation can be used to determine the pressure dropfor the liquid only section, with liquid viscosity μ_(l), length ofliquid piping L_(liq), volumetric flow rate V, and pipe inner diameterD_(i), as provided in:

$\begin{matrix}{{\Delta P_{liq}} = {128\frac{\mu_{l}L_{liq}V}{\pi D_{i}^{4}}}} & \left( {{EQ}.\mspace{11mu} 6} \right)\end{matrix}$

Pressure Drop in Two-Phase Section: For predicting the pressure drop inthe two-phase section, the separated flow model shown in EQ. 7 below wasused, with the Lockhart and Martinelli correlation for determining voidfraction and the Friedel correlation for determining the two-phasemultiplier. (See, e.g., Carey, V. P. Liquid-vapor phase-change phenomena(1992); Lockhart, R. & Martinelli, R. Chem. Eng. Prog 45, 39-48 (1949);Friedel, L. In European two-phase flow group meeting, Paper E, vol. 2,1979 (1979), the disclosures of which are incorporated herein byreference.)

$\begin{matrix}{\frac{\Delta P}{\Delta z} = {- {\frac{1}{\Lambda}\left\lbrack {{\left( \frac{dP}{dz} \right)fric} + {\left( \frac{dP}{dz} \right){grav}} + {\left( \frac{dP}{dz} \right){acc}}} \right\rbrack}}} & \left( {{EQ}.\mspace{11mu} 7} \right)\end{matrix}$

Wick Characterization: In attempts to achieve the goals of obtainingsurface isothermality and liquid spreading for improved wetting of theheated surface, the use of a wick element for its capillary action isexplored. Unlike in heat pipes where the wick provides the drivingpotential of the entire cooling loop, here the wick's purpose is to moreeffectively supply liquid to locations of discrete heat loads, wherevapor concentration would otherwise take place. The important wickcharacteristics are the porosity e, the permeability K, and the poresize r_(eff). The evaporator was designed around a 5.5″×6.5″×0.25″sintered stainless steel 316L wick, with an equivalently sized HardOff-White S2 Felt wick from McMaster also to be tested.

Porosity: To estimate the porosity, the procedure presented in Holleyand Faghri may be used. (See, e.g., Holley, B. & Faghri, A. Appliedthermal engineering 26, 448-462 (2006), the disclosure of which isincorporated herein by reference.) The wick was placed on a scale andacetone was supplied to the wick until it reached saturation. By takingvideo while slowly filling the wick, the amount of mass at the time ofsaturation could be determined. Using this mass and the fluidproperties, the volume fraction of pores can be found using EQ. 8,

$\begin{matrix}{\frac{1}{ɛ} = {1 + \frac{V_{wick}\rho_{a}}{m_{a}}}} & \left( {{EQ}.\mspace{11mu} 8} \right)\end{matrix}$

where V_(wick) is the total volume of the wick sample, ρ_(a) is thedensity of acetone and m_(a) is the amount of mass added at the time ofsaturation. The values are presented in the Table 1, below.

TABLE 1 Property Stainless Steel Wool Felt Rated Pore Size (μm) 60 12Pore Size (μm) 59 ± 10 14 ± 2  Rated Permeability (×10⁻¹⁰m²)* 0.05-15  N/A Permeability (×10⁻¹⁰m²) 0.4 ± 0.1 0.8 ± 0.2 Rated Porosity*0.28-0.35 N/A Porosity 0.34 ± 0.03 0.36 ± 0.04

Pore Size and Permeability: To solve for the pore size and permeabilityof a given wick, a rate of rise test was performed, as also outlined inHolley and Faghri, cited above. This entailed dipping a wick sample intoacetone and measuring the amount of liquid that enters the wick overtime. If the wick is light colored, this can be accomplished simply byvisually observing the height of the wicked fluid as it rises. Howeverin many wicks, especially metal samples, it is difficult to visualizethe liquid level in the wick, so the mass is measured over time. In therate of rise test, the capillary pressure ΔP_(c) of the pores worksagainst the gravitational head ΔP_(g) and the viscous pressure dropΔP_(w) through the wick according to:

ΔP _(c) =ΔP _(w) +ΔP,   (EQ. 9)

Using the Young-Laplace equation, Darcy's Law, and hydrostatic pressure,EQ. 9 can be manipulated into EQ. 10, which is to be used if measuringthe mass rate of change by way of taking video of a scale reading.Parameters include m as the mass of acetone over time, a as the surfacetension of acetone, A as the cross sectional area, and g as theacceleration due to gravity.

$\begin{matrix}{\frac{dm}{dt} = {{\frac{2\sigma}{r_{eff}}\frac{K{ɛ\left( {A\rho_{a}} \right)}^{2}}{\mu_{l}}\frac{1}{m}} - \frac{g\rho_{a}^{2}AK}{\mu \iota}}} & \left( {{EQ}.\mspace{11mu} 10} \right)\end{matrix}$

If measuring the height of the liquid line over time, by substitutingthe liquid height x =m/(ρ_(a)εA), the expression reads as follows:

$\begin{matrix}{\frac{dx}{dt} = {{\frac{2\sigma}{r_{eff}}\frac{K}{\mu_{l}ɛ}\frac{1}{x}} - {\frac{K}{\mu_{l}ɛ}\rho_{l}g}}} & \left( {{EQ}.\mspace{11mu} 11} \right)\end{matrix}$

Therefore, by measuring the slope and the intercept of the time rate ofchange versus reciprocal plot, and by knowing the fluid properties andporosity, the permeability and pore size can be estimated using theabove equations. The values obtained are presented in Table 1. The dataare presented in FIGS. 15a to 15 d.

EXAMPLE 2 Mixed Flow Architecture Tests

MFA systems are the typical mechanically pumped two-phase systems. Theyare commercially available for ground-based systems and have also beendeveloped for use in spacecraft. An MFA is currently being used for thethermal control system of an ISS science payload. (See, e.g., Delil, A.A. M., A. A. Woering, and B. Verlaat. Development of a MechanicallyPumped Two-Phase CO2 Cooling Loop for the AMS-2 Tracker Experiment. No.2002-01-2465. SAE Technical Paper, 2002, the disclosure of which isincorporated herein by reference.) In these systems, the pump,evaporator and condenser are plumbed in series in a single circuit (seeFIG. 9). Liquid flows from the pump to the evaporator where heat isadded to form a two-phase flow. A pre-heater at the inlet of theevaporator is often used to bring the working fluid up to the saturatedstate. Heat is rejected from the flow in the condenser, which outputssub-cooled liquid back to the pump. The evaporator typically utilizessome form of flow boiling.

FIG. 16 schematically shows a plot of pressure versus position for abasic MFA system. The pressure monotonically decreases from the pumpoutlet to the pump inlet. The largest pressure drop is in the portion ofthe system with two-phase flow (from evaporator inlet (2) to condenseroutlet (5)). The pressure drop in the liquid lines is relatively smallsince flow rates are relatively low and the flow is single phase.

Mixed Flow Testbed

In order to experimentally evaluate an MFA system, a simple MFA testbedwas designed and built. The goal of the testbed was to enableexperimental studies to be carried out in order to evaluate theoperation of embodiments of the evaporator in an MFA thermal transfersystem. FIG. 17 shows a schematic of the system. The working fluid wascirculated in a closed loop through a pre-heater, evaporator andcondenser. An accumulator was used to regulate system pressure. Thepre-heater consisted of electrical heaters mounted to a cold plate. Aflat plate liquid-liquid heat exchanger connected to a chiller was usedas the condenser/subcooler. The accumulator consisted of a samplecylinder located at the high point of the physical system. The samplecylinder was partly filled with liquid when the system was charged; itsorientation with respect to gravity and high location, ensured that theaccumulator remained filled and able to regulate system pressure.Accumulator pressure was either controlled by pre-pressurizing theaccumulator with air, or by evacuating the vessel and operating it as asaturated liquid/vapor system. In this case the accumulator pressure wascontrolled by controlling the temperature using a heater/controllercircuit. Plumbing to the accumulator was arranged such that it could beused to fix the pressure either near the evaporator inlet or near thepump inlet. Typically, the accumulator is placed at the pump inlet toensure that the NPSH requirement is met, however it was thought that byplacing it near the evaporator, temporal thermal stability might be ableto be better enforced. A gear pump was used to circulate the fluid.

The loop was instrumented to read fluid temperatures and pressures inmultiple locations. Flow rate was measured near the pump outlet. Sightglasses installed at the evaporator inlet and outlet were used tovisually monitor the thermodynamic quality of the flow. All transportlines were made of 6.35 mm (0.25 in) stainless steel pipe. Water wasused as the working fluid for simplicity. The evaporator described abovewas used in the testbed.

MFA Test Campaign

The MFA testbed was used to experimentally explore the potential of anMFA system and to characterize the evaporator design according toembodiments. Multiple parameters were varied including flow rate,transport line hydraulic resistance, and evaporator orientation withrespect to gravity. Flow rate was modulated via changing the pump speed,transport line resistance was varied using an inline needle valve, andthe level of preheat was varied using electrical heaters. Changing theevaporator orientation required reinstalling the evaporator in a neworientation. Changing the accumulator position entailed opening andclosing the correct valves.

During the course of the test campaign, operational settings were foundthat yielded a fairly isothermal evaporator for a range of heat loads.However, to achieve this required significant heat input in thepre-heater to bring the fluid close to saturation. It was determinedthat the evaporator was unable to maintain a tolerable level ofisothermality if the fluid at the inlet was significantly subcooled.

Several other findings were made in the course of testing: It was foundthat the accumulator can produce flow instabilities if it is locatednear the evaporator inlet, and that the evaporator was able to operatewith the vapor side facing either down or up. A more detailed discussionof these findings is given below. Throughout the testing, systempressure was maintained between 4 psia and 6 psia to ensure sufficientNPSH for the pump. The liquid return line (exiting thecondenser/subcooler) was always subcooled to 20° C.

Steady State Operation with an Isothermal Evaporator. System parameterswere adjusted in order to try to maintain an isothermal evaporator witha range of heat loads, in accordance with the program goals. Theaccumulator was positioned at the evaporator inlet and flow resistanceupstream of the evaporator was increased (using the needle valve) toensure flow stability. The accumulator pressure was set by reducing theair pressure in the headspace of the accumulator to about 2.5 psia. Theflow rate was fixed at 50 mL/min. 140 W were applied to the pre-heaterto bring the fluid temperature from about 21° C. up to 60° C. at theevaporator inlet. Four different heat loads were sequentially applied tothe evaporator: 200 W, 250 W, 300 W and 395 W. The heater measuredapproximately 6.5 cm by 15 cm and covered about 17% of the evaporatorsurface. The evaporator was oriented vapor side down. In principle, thisis the more challenging orientation for isothermality since the vaporwill tend to rise away from the heated surface due to buoyancy. Keytemperatures and pressures were recorded around the loop, and IR imagesof the evaporator were continually acquired.

FIGS. 18a and 18b show select temperatures and pressures in the loopover the course of the experiment, as well as the heat loads applied tothe evaporator. The pump was turned on at about 0.25 hr. At 1.25 hr, 140W were applied to the pre-heater. At 2.9 hr, the evaporator heater waspowered to 200 W. The evaporator heat load was subsequently increased to250 W (5 hr), 315 W (7.1 hr) and then 390 W (7.25 hr).

Over the course of the experiment, the accumulator pressure increasedboth gradually and suddenly. The pressure increases are mostly due tothe volume of air in the accumulator being compressed as the fluid inthe loop expands. The primary fluid expansions occur when the pre-heateris turned on (1.3 hr), when the initial heat load is applied to theevaporator (2.9 hr), and when the final heat load is applied to theevaporator (7.25 hr). The evaporator heat load produces vapor, whichleads to a significant increase in working fluid volume. Superimposed onthese relatively sudden pressure changes is a more gradual increase inpressure. This is due to the temperature of the gas in the accumulatorgradually rising over the course of the test. This was due to theambient room temperature gradually increasing. The ambient temperaturerise is also reflected by the gradual increase in the fluid temperatureat the outlet of the condenser/subcooler. These pressure increases areattended by an increase in saturation temperature. This explains theslight increase in the evaporator temperature over the course of thetest. For the majority of the test the fluid at the evaporator inlet wasapproximately 9° C. subcooled.

The pressure drop remained fairly constant over the course of the testwith a slight decrease when the pre-heater was activated and an increasewhen the evaporator heat load was increased. The initial decrease near 2hr was due to a reduction in fluid viscosity as the fluid warmed. Thesubsequent increases in AP were due to the two-phase pressure drop whenthe evaporator heater was activated or changed.

The most noteworthy characteristic of the data is that the fluidtemperature at the outlet of the evaporator does not significantlychange as the heater power is nearly doubled. This is one of thewell-touted attributes of mechanically pumped two-phase systems. IRimages of the evaporator illustrate this phenomenon even more clearly.FIG. 19 shows IR images of the evaporator with three different heatloads. In each case the evaporator is effectively the same temperature.These images also highlight the high degree of isothermality attained onthe evaporator. The entire surface is within a temperature band of 3°C., although it should be noted that the evaporator is aluminum (highlyconductive) and about ¼ of the target evaporator size. A single phasesystem using the same working fluid and flow rate would have atemperature difference of 111 C across a heat load of 390 W (using:Q=mc_(p)ΔT).

Subcooling Study: One of the concerns with an MFA system is that someform of pre-heater must be used to bring the working fluid up near thesaturation temperature. This is done to: (1) ensure two-phase heattransfer in the evaporator and (2) to keep the temperature differenceacross the evaporator as low as possible. Ideally, the entire evaporatoris at the saturated temperature, which varies little over its length.The isothermal capability of the evaporator described above wasexperimentally investigated by applying several different heat loads anddegrees of subcooling. As would be expected, the degree of isothermalityis a strong function of the heat load and degree of subcooling.

In the test case described above (steady state isothermal evaporator), afairly isothermal evaporator was obtained with a heat load varying from200 W to 390 W and the inlet fluid being subcooled by 9° C. The heaterwas centered on the evaporator in this case and covered approximately17% of the heat acquisition surface. A more challenging case is toachieve isothermality when there is a high degree of subcooling, or asmall heater is used with a low heat load. This is the case that isdiscussed below.

In an ideal evaporator, any heat load would produce a layer of vapor,which would fill the evaporator's vapor cavity. This would force theentire heat acquisition surface to be near the saturation temperature.This was the goal with embodiments of the MFA evaporator. In thisexperiment, however, several issues were identified that inform optimalevaporator design.

First, in this exemplary embodiment the casing was made from a materialwith a high thermal conductivity (aluminum). The high conductivity makesit hard to decouple the evaporator liquid temperature from the heatacquisition surface, which results in the unwanted cooling of thatsurface. Accordingly, in embodiments a casing material that has a lowerthermal conductivity may be used. The other oversight is morefundamental and related to fluid dynamics. The issue arises from thefact that the liquid and vapor chambers are not completely separated.The liquid can flow into the vapor side either through the wick (ifthere is no liquid vapor interface in the wick to inhibit flow) orthrough the connecting channel. This was observed to occur duringtesting and inhibited the ability to maintain an acceptable degree ofisothermality when subcooled liquid was present. An additional issuecame from the vapor produced by the heat load evidently gettingsuperheated.

A series of tests was conducted where a 2.4 cm×10 cm heater mounted nearthe evaporator inlet was used to supply 48 W of heat. The heater coveredapproximately 10% of the heat acquisition surface. The isothermality ofthe surface was measured as a function of different levels of subcoolingat the evaporator inlet. The degree of subcooling was varied from 10 to0.5° C. The isothermality of the evaporator was measured using thetemperature difference between a point right next to the heater and apoint close to the edge of the vapor cavity by the evaporator outlet.Temperatures were measured using an IR camera. FIG. 20 shows an IR imageof the evaporator with the line along which temperature was measured.While the temperature gradient decreased as the level of subcoolingdecreased, it remained appreciable. Even with only 0.5° C. ofsubcooling, the ΔT across the evaporator was found to be 6° C. In thecase where the subcooling was 10° C., the ΔT across the evaporator wasalso 10° C. The temperature gradient was due to two components: (1) TheΔT between the sub-cooled liquid and saturated vapor, and (2)superheating of the vapor near the heater. The latter reason caused theΔT across the evaporator to be even larger than the degree ofsubcooling, when the subcooling was relatively small.

Accumulator Position: In mechanically pumped two-phase fluid loops, theaccumulator is often located near the inlet of the pump, to ensuresufficient NPSH. However, in principle, locating the accumulator thisfar from the evaporator could be seen to be sub-optimal for a systemthat requires high temporal stability in the evaporator. After all, theoperating temperature in the evaporator is ultimately dictated by itssaturation temperature, which is in turn controlled by the localpressure. The accumulator provides the control point for the pressure inthe loop. Accordingly, in embodiments the accumulator is disposed closerto the evaporator, such that it can be used to more directly regulatethe evaporator pressure and thus enhance its temperature stability.

As such, MFA system testing was done with the accumulator in twodifferent positions: (1) near the evaporator inlet or (2) near the pumpinlet. It was found that locating the accumulator near the inlet of theevaporator led to flow instabilities in the form of periodicoscillations in the fluid flow. It was found that these oscillationscould be suppressed by increasing the flow resistance between the inletof the evaporator and the accumulator. This somewhat negated the benefitof situating the accumulator near the evaporator, since the additionalresistance inherently creates a separation in pressure between theaccumulator and evaporator. Additional details are given below.

When the accumulator was located at the inlet of the evaporator, a flowinstability arose soon after a heat load was applied to the evaporator.The flow instability manifested as a periodic oscillation where thesystem alternated between two different states. In the primary stateoperation was normal and the fluid flowed in the expected manner; in thesecondary state the fluid would flow backwards in the section of theloop between the accumulator and evaporator. That is, the fluid wouldflow from the evaporator to the accumulator. This was clearly observedin the experiment as vapor could be seen entering the accumulator via asight glass.

This phenomenon can be explained using a simple flow resistance model.FIG. 21 shows a schematic of the MFA system with the flow resistances.During normal operation the pressure monotonically decreases from thepump outlet to the pump inlet (P1>P2>P3>P4>P5). However, the instabilitysets in when the two-phase pressure flow resistance (R2) starts to growas the flow is established. The growing two-phase flow restrictioncauses the pressure in the evaporator to build as additional vapor iscontinually produced by the heat load. At some point, the pressure inthe evaporator (P3) becomes larger than the pressure in the accumulator(P2), and it becomes easier for the fluid to flow to the accumulator.Analytically, this corresponds to the case when (P3-P2)/R1>(P3-P4)/R2.During the period of backflow, the pumped fluid is redirected into theaccumulator. This backflow condition can be suppressed by increasing theresistance between the accumulator and the evaporator (R1) such that(P3-P2)/R1<(P3-P4)/R2 always.

Evaporator Orientation: The exemplary evaporator designed for thistestbed incorporates a wick element to mitigate the effects of gravity.One of the primary functions of the wick is to ensure that the heatedsurface stays wetted, regardless of orientation. Dry out of the heatedsurface is a known issue, especially if the heated surface is facingupwards. Testing was done in order to verify that the wick wasfunctioning as anticipated and was able to keep the heated surfacewetted, without vapor penetration. To do this, the evaporator wasoperated both with the heated surface facing downwards and with theheated surface facing upwards. Results indicated that the wick elementfunctioned as anticipated. Dry out was never encountered with the heatedsurface facing upwards and heat fluxes up to 2.5 W/cm². Additionally,the heated surface was kept within 3° C. of the fluid saturationtemperature when the heated surface was facing downwards, with subcooledliquid entering the evaporator (see FIG. 20). The heat load was at least200 W and level of subcooling was 9° C. This suggests that the wickprovided effective phase separation when the heat load was high enough.

EXAMPLE 3 Separated Flow Architecture Tests

In other embodiments, an SFA architecture was explored. FIG. 9 (left)shows the key elements of the architecture. A pump circulates theworking fluid, an evaporator absorbs the heat load, and a condenserrejects the heat load. An accumulator is used to set the system pressureat the pump inlet. The evaporator is designed similarly to a CPLevaporator, with liquid and vapor channels being separated by a porouswick (FIG. 10a ). Unlike a CPL evaporator, the SFA evaporator has aliquid outlet line that allows the liquid flow to bypass the evaporatorand continually circulate during normal operation. This means thatduring normal operation, excess liquid is not forced through the wick bythe pump. Instead, the wick picks up whatever liquid it needs to satisfythe vapor mass flow rate required by the heat load. The vapor and liquidphases remain separated in the entire loop except for in the condenser.

To better understand the architecture, it is useful to consider howpressure varies through the system at steady state. FIG. 22schematically shows the pressure in the system as a function ofposition. These positions correspond to the locations labeled in FIG. 9.At the outlet of the pump (1), the pressure is high. Between the pumpoutlet and evaporator inlet the flow is single phase liquid and pressuredecreases monotonically. Inside the evaporator, two distinct pressureregions exist: one in the vapor chamber (2′) and one in the liquidchamber (2). These chambers are completely separated by a porous wickthat contains the liquid-vapor interface during steady state operation.The application of a heat load maintains the presence of vapor in thevapor chamber. The liquid-vapor interface forms a meniscus that cansustain a pressure difference across it. During normal operation, thepressure in the vapor chamber is higher than in the liquid chamber. Thisprevents liquid from being forced into the vapor chamber by the pump.Depending on how the system is designed, the vapor pressure in the vaporchamber can be even higher than at the pump outlet. The pressure in theliquid chamber varies relatively little between the inlet and outlet ofthe evaporator, since the hydraulic diameter here would typically bebigger than in the transport lines. In the liquid bypass line betweenthe evaporator (2) and the point where the liquid and vapor lines meet(5), the pressure drops monotonically due to the flow of liquid. In thevapor line between the outlet of the evaporator (2′) and the point wherethe two flows meet (3), the pressure also monotonically decreases. Inthe first leg of the line between the evaporator (2′) and condenser (3),the flow is pure vapor; in the second section within the condenser (3 to4) the flow is two-phase; and in the third section from the condenseroutlet (4) to the point where liquid and vapor lines recombine (5) theflow is pure liquid. After the two lines meet the flow is liquid up tothe pump inlet (6). The pressure at the pump inlet is fixed by theaccumulator.

Basic Model

In this section a basic lumped parameter model is developed and used tobetter understand how an SFA system operates in accordance withembodiments and what some of its limitations are. Consider the portionof a simple SFA system from the evaporator inlet to the point after thecondenser where the liquid and condensed vapor lines meet (point 6 inFIG. 9). This is the section of an SFA system that is of primaryinterest. FIG. 23 shows two schematics of this section illustrating thephysical system and its simplified, abstracted circuit diagram. Atsteady state, the vapor chamber is filled with vapor as is the linebetween the outlet of the evaporator and the condenser. The condensercontains two-phase flow, and the remainder of the system containsliquid. The liquid and vapor phases are separated in the evaporator by ameniscus, which forms in the wick (just as in a heat pipe). The keyphysical parameters accounted for in the lumped parameter model areshown on the circuit diagram. Pressure in the liquid chamber and oneither side of the meniscus is captured as well as flow resistances inthe wick (R_(wick)), liquid chamber (R_(liq)) and vapor chamber(R_(vap)). Note that and also include the hydraulic resistances of theliquid and vapor lines at the outlet of the evaporator up to the pointwhere the two lines meet (point 2 in FIG. 23).

At steady state operation the system can be described with the followingequations:

P ₍ ₎−pressure Q={dot over (m)} _(vap)λ  (EQ. 12)

{dot over (m)}₍ ₎−mass flow rate {dot over (m)} _(tot) ={dot over (m)}_(vap) +{dot over (m)} _(lip)   (EQ. 13)

Q−heat load on evap P ₁ −P ₂ ={dot over (m)} _(liq) R _(liq)   (EQ. 14)

R ₍ ₎−flow resistance P ₃ P ₂ ={dot over (m)} _(vap) R _(vap)   (EQ. 15)

λ−latent heat of vap. P ₁ −P′ ₁ ={dot over (m)} _(vap) R _(wick)   (EQ.16)

The variables are defined next to the equations and are shown in FIG.23. EQ. 12 relates the heat applied to the evaporator to the rate ofvapor formation. Implicitly, this equation only accounts for the heatthat goes into the vapor-eat that goes into bringing the subcooledliquid up to saturation and heat losses are not included. EQ. 13 statesthe conservation of mass for the system. EQ. 14 describes therelationship between pressure drop and flow rate through the liquidchamber of the evaporator and the entire liquid line up to the pointwhere it recombines with the condensed vapor line. EQ. 15 describes thepressure drop/flow relationship from the vapor side of the meniscusthrough the condenser up to where the two flow lines meet. EQ. 16describes the hydraulic flow through the wick from the liquid chamber upto the liquid side of the meniscus. Depending on the flow regime, theflow resistance may be a function of the flow rate. The pressure dropbetween the inlet of the evaporator and the liquid side of the wick areconsidered negligible. The model is a lumped-parameter model thatassumes the steady-state operation described above.

To apply the model, the dependent and independent variables must bedefined. For an actual system, it would be expected that the total flowrate (m_(tot)) is known and controlled. With this information andknowledge of the system, the pressure at the inlet of the evaporator(P₁) is easily found. Similarly the hydraulic resistances of the systemwould be dictated by the physical geometry and the heat load (Q) wouldbe known. The latent heat of vaporization (λ) would be fixed by theselection of a working fluid. This implies that the independentvariables of the model are: m_(tot), λ, Q, R_(liq), R_(vap), andR_(wick). The dependent variables are then m_(vap), m_(liq), P₂, P₃, andP′₁. With these stipulations, EQs. 12 to 16 form a system of 5 equationswith 5 unknowns. The problem is therefore well posed and has a uniquesolution.

The model given by EQs. 12 to 16 can be used to explore how changes inthe independent variables (e.g. m_(tot)) affects system performance. Ofparticular interest is to see how an SFA system compares to a CPL or LHPtype system. The fact that the system relies on the capillary head ofthe wick for operation begs the question of whether this architectureoffers any advantage to a CPL or LHP. In a CPL and LHP, the evaporatorfunctions as the pump, and pressure increases across the meniscus fromthe liquid to the vapor side. This pressure rise adjusts so that it isequal to the pressure drop in the remainder of the system. One of theclassical limits of a CPL/LHP is that the pressure drop in the systemmust be less than the capillary head available in the capillary pum3.For a capillary pump, the maximum available head is equal to: σ/r_(eff).This cap on the max allowable pressure drop ultimately limits theallowable heat load, since increasing the heat load increases the massflow rate, which increases the pressure drop. Using the model developedabove (EQs. 12 to 16), the pressure difference across the meniscus(P₃−P′₁) can be solved for in terms of the independent (known)quantities. Doing this yields:

$\begin{matrix}{\left( {P_{3} - P_{1}^{\prime}} \right) = {{\frac{Q}{\lambda}\left( {R_{vap} + R_{liq} + R_{wick}} \right)} - {{\overset{.}{m}}_{tot}R_{liq}}}} & \left( {{EQ}.\mspace{11mu} 17} \right)\end{matrix}$

Note that unlike an LHP, the pressure across the meniscus is not simplyequal to the pressure drop across the system—it is a more complexfunction of flow resistances, the pumped flow rate and heat load. EQ. 17suggests that even the pressure drop across the vapor line is not solelyreliant on the capillary head developed across the meniscus. This can bemore clearly seen by substituting EQ. 12 into EQ. 17 and solving for thepressure drop in the vapor line (P₃−P₂):

$\begin{matrix}{\left( {P_{3} - P_{2}} \right) = {\left( {P_{3} - P_{1}^{\prime}} \right) + {{\overset{.}{m}}_{tot}R_{liq}} - {\frac{Q}{\lambda}\left( {R_{vap} + R_{wick}} \right)}}} & \left( {{EQ}.\mspace{11mu} 18} \right)\end{matrix}$

For a given heat load (Q), the vapor mass flow rate is fixed as is thepressure drop across the vapor line (m_(vap)). The pressure headdeveloped to overcome this pressure drop comes from the meniscus(P₃−P′₁) and the pump (m_(tot)R_(liq)). The burden on the meniscus canbe reduced by increasing the mass flow rate put out by the mechanicalpump (m_(tot)), or by increasing the resistance in the liquid line(R_(liq)). This second effect can be more clearly seen by substitutingEQs. 12 and 13 into 18 and rearranging:

(P ₃ −P ₂)=(P ₃ −P′ ₁)+{dot over (m)} _(lip) R _(lip) −{dot over (m)}_(vap) R _(wick)   (EQ. 19)

While an SFA system is not as reliant on the capillary pumping head asan LHP, it still has limits of operation that stem from the capillarywick in the evaporator. For normal SFA operation, the vapor and liquidin the evaporator must remain separated by the meniscus in the wick.This means that liquid cannot flow into the vapor chamber, and similarlyvapor cannot flow into the liquid chamber. In order for liquid to beprevented from flowing into the vapor chamber, the pressure must behigher in the vapor chamber than in the liquid chamber. However, inorder to ensure that vapor does not penetrate the wick and enter theliquid chamber, the pressure across the meniscus cannot exceed theavailable capillary pressure head: 2σ/r_(eff). If the availablecapillary pressure is exceeded, vapor will push back the meniscus andflow into the liquid chamber. These requirements on pressure can beformalized as:

$\begin{matrix}{0 < \left( {P_{3} - P_{1}^{\prime}} \right) < \frac{2\sigma}{r}} & \left( {{EQ}.\mspace{11mu} 20} \right)\end{matrix}$

This equation states that the pressure difference across the meniscusmust be less than the maximum available capillary head and greater thanzero. Substituting EQ. 17 into EQ. 20 and rearranging yields limitationson the allowable heat load (Q) for a given system:

$\begin{matrix}{\frac{\lambda \left( {{\overset{.}{m}}_{tot}R_{liq}} \right)}{\left( {R_{vap} + R_{liq} + R_{wick}} \right)} < Q < \frac{\lambda \left( {\frac{2\sigma}{r} + {{\overset{.}{m}}_{tot}R_{liq}}} \right)}{\left( {R_{vap} + R_{liq} + R_{wick}} \right)}} & \left( {{EQ}.\mspace{11mu} 21} \right)\end{matrix}$

If the heat load is less than the minimum allowable value given by theleft hand side of EQ. 21, liquid will enter into the vapor chamber; ifthe heat load is greater than the maximum allowable value given by theright hand side of EQ. 21, vapor will enter the liquid chamber. Asalluded to previously the maximum allowable heat load is not solelylimited by the available capillary head of the wick 2σ/r_(eff) as in anLHP. Instead, it is also a function of the hydraulic resistances in thesystem, the latent heat of the working fluid, and the mass flow rateproduced by the pump. The max allowable heat load can be increased in afew different ways: by decreasing hydraulic resistances in the system orby increasing the mass flow rate put out by the pump. This gives the SFAa system level advantage over an LHP or CPL: the max allowable heat loadis not solely dictated by the capillary wick.

With some clear limitations on system performance defined by the model,it is natural to ask how a real system might be limited. To do this, themaximum and minimum allowable heat loads were calculated for a specificset of system parameters (line length, fluid properties, flow rate). Thesystem was assumed to have a liquid line of 5 m (from the evaporatorliquid outlet to the point where the liquid and vapor line recombined);the vapor line was assumed to be 15 m (from the evaporator vapor outletthrough the condenser up to the point where the liquid and vapor linesmeet). The internal diameter of all lines was assumed to be 9.52 mm(0.375 in). The effective pore size of the wick was assumed to be 60 umand the permeability was assumed to be 4×10⁻¹¹ m². Fluids were assumedto be perfectly wetting. Hydraulic resistances were calculated usingeither a laminar or a turbulent model as appropriate. The flow rate washeld constant at 200 mL/min. Fluid property data was taken from NISTreference values assuming an operating temperature of 20° C. The resultsfor 26 different fluids are shown in Table 2.

TABLE 2 Fluid Q_(min) (W) Q_(max) (W) Ammonia 217.6 1559.2 Butane 103.4438 1-Butene 101.4 450.2 Deimethylether 109.5 568.5 Hydrogen sulfide155.2 861.1 Isobutane 111.8 416.2 Isobutene 102.6 449.2 Propane 141.8492.1 Propylene 147.8 525.5 Propyne 126.5 587.7 R115 67.8 278.2 R12 70.1383.1 R1234 74.8 344.1 R1234ZE 80.1 411 R124 125.5 542.6 R134A 86.7427.1 R142 104.4 507.6 R143 90.4 351.8 R152A 99.2 492.2 R161 116.2 565.1R218 65.1 245.8 R22 85.5 460.5 R227EA 95.1 402.5 R32 104.5 518.4 Sulfurdioxide 136.5 953.5 Water 9.2 65.3

Of course other practical limitations may constrain an SFA system morethan the constraints defined in EQ. 21, however these constraints givean initial filter to see how a real SFA system using different fluidsmight be limited. In Table 1, several fluids look promising from anallowable heat load perspective including some common refrigerants andheat pipe fluids such as ammonia and propylene. Surprisingly water doesnot offer very good performance. This is because the latent heat is notthe only property that matters—other properties that enter into thehydraulic resistance are important as well (liquid and vapor viscosityand density).

Separated Flow Testbed

A testbed was designed and developed and built to experimentally exploreembodiments of a SFA thermal control system incorporating embodiments ofthe evaporator. A schematic of the SFA testbed is shown in FIG. 24. Acentrifugal pump circulates the working fluid sequentially through apre-heater, evaporator, condenser, and subcooler. The flow is split intotwo parallel paths in the evaporator where the heat load is applied. Onepath is used for vapor transport and the other for transporting liquid,which bypasses the evaporator. After the vapor flow is condensed in thecondenser, the two streams reunite to pass through the subcooler. Othercomponents include a gas trap to capture non-condensable gases, a filterto protect the pump, and an accumulator to control system pressure. Theplumbing for the accumulator was setup so that the accumulator positioncould be changed between the evaporator inlet and the pump inlet.Multiple needle and ball valves were incorporated to facilitatemaintenance operations and vary operational conditions. The system wasinstrumented to read fluid temperatures, pressures and flow rates inmultiple locations around the loop. Water was used as the working fluidfor convenience.

Most of the components used were commercial off-the-shelf parts. Aminiature centrifugal style pump was used to circulate the fluid. A highefficiency cold plate mounted with heaters was used as the pre-heater.Brazed plate heat exchangers were used as the condenser and subcooler. Acustom accumulator was built to enable temperature control for saturatedfluid pressure regulation. The accumulator was essentially a shell andtube heat exchanger, where the shell side was attached to the SFA systemand the tube side circulated fluid from a temperature controlledchiller. This enabled the temperature control of the accumulator, whichalso dictated pressure for this saturated system. The system wasinsulated to thermally isolate it from the environment. The evaporatordescribed above in accordance with embodiments was used in the testbed.

SFA Test

A set of tests were carried out utilizing the testbed described above.Operational parameters were varied such as: flow rate, evaporator heatload, degree of pre-heating, and the method of pressure control in theaccumulator. The general goal of the testing was to develop anunderstanding of how the system behaves utilizing an evaporator inaccordance with exemplary embodiments. Additionally, the systemstability was investigated by perturbing the system and observingwhether it could regain stable operation. The condenser temperature andheat loads were varied along with the pump operation. Over the course oftesting the following points were demonstrated:

-   -   A stable separated flow system is possible;    -   A pre-heater is not required for an SFA system; and    -   The SFA system is stable and robust;        -   Evaporator performance was insensitive to fluctuations in            condenser temperature of 10° C. (0.8° C./min rate), and        -   The system could recover from excessive/insufficient heat            loads, the pump stopping, and the condenser stopping to            function for temporary periods.

Nominal Operation: Stable system operation with the exemplary SFAtestbed was demonstrated for two different flow rates and a range ofheat loads. It was found that the pre-heater was not needed. Duringstable operation, separated flow was established in the system withvapor filling the vapor line and liquid filling the liquid line. Stable,separated flow was only achievable for a range of heat loads. If theheat load was too low, liquid was seen in the vapor line; if the heatload was too high, vapor was seen in the liquid line. Additionally, itwas found that higher flow rates could accommodate higher heat loads.This is also consistent with the theory presented above. Table 3 showsthe range of allowable heat loads established for two different flowrates for the SFA testbed using water as the working fluid.

TABLE 3 Flowrate (mL/min) Range of heat loads (W) 50 225-250 79 275-350

FIG. 25 shows test data from the SFA testbed at stable operation. Theflow rate was 79 mL/min and no pre-heater was used. System pressure washeld at 5 psia at the pump inlet using a temperature controlled,saturated accumulator. The evaporator saturation temperature was 68° C.;the subcooler was set to output fluid at 21° C. The evaporator heat loadwas set to 350 W for the entire test except from 2.25 hr to 4 hr, whenthe heat load was 275 W. The flow remained separated over the course ofthe test. This was verified by observing sight glasses placed at theevaporator outlets, and was corroborated by the fact that the evaporatorliquid outlet temperature was subcooled, while the vapor outlettemperature was saturated.

Promising features of this data are: (1) The temperature at theevaporator outlet does not significantly change with heat load, asexpected; (2) the flow remains separated with the liquid at the liquidoutlet of the evaporator remaining subcooled and the vapor at the vaporoutlet remaining saturated; (3) No pre-heat is needed for stableoperation, even with the fluid entering the evaporator 50° C. subcooled.These features demonstrate the stable operation of the SFA testbed, andits potential to provide performance without a pre-heater.

Other features of the data are: (1) the liquid on the liquid side of theevaporator increases in temperature by about 45° C. from the inlet tothe outlet, and (2) the outlet vapor temperature becomes a little morenoisy when the lower heat load is applied (275 W). The increase intemperature of the liquid flow in the evaporator indicates significantheat transfer to the liquid and that the liquid and vapor sides arethermally well-coupled. Using Q=(mc_(p)ΔT), the heat loss during thisexperiment was estimated to be 173 W or 49% of the applied heat load.This strong thermal connection between the liquid and vapor sidesinhibits the performance of the evaporator in two ways. Firstly, itprevents the evaporator from reaching any reasonable degree ofisothermality for small heat loads, if the fluid is entering highlysubcooled. In this case, the temperature of the cold liquid cools thetop surface of the evaporator near the liquid inlet and the temperaturegradient across the heat acquisition surface becomes untenable. In thesame vein, the heat that is lost to the liquid side would otherwise beused to produce more vapor, which would make it more likely to fill thevapor cavity with vapor. Filling the vapor cavity with vapor is apre-requisite for an isothermal evaporator. Secondly, when the heattransfer path to the liquid is too large, boiling can occur in theliquid side of the evaporator. This can prematurely limit the maximumallowable heat load that would otherwise be possible. For these reasonsdecoupling the liquid and vapor regions is crucial for good performanceof the evaporator.

System Stability: The stability of the SFA testbed was investigated byestablishing a stable operating condition (with stable separated flow),and perturbing it. Methods of perturbing included: stopping the pump,varying the condenser temperature, exceeding the max allowableevaporator heat load, or applying a heat load below the minimumallowable heat load for stable operation. In all these cases, the systemwas able to consistently recover once the nominal operating conditionswere restored.

FIG. 26 shows test data incorporating four different perturbations.Steady separated flow was initially established in the testbed with aflow rate of 79 mL/min and an evaporator heat load of 350 W. Theaccumulator was located at the pump inlet. After stable operation wasestablished (0.8 hr), the perturbation sequence was initiated. Thisconsisted of forcing a known operating condition that resulted in theloss of steady separated flow and then restoring the initial nominaloperating conditions. In each case, the testbed reverted to its initialstable state once the nominal operating conditions were restored. At 1hr the evaporator heat load was increased to 450 W—this resulted in theloss of separated flow with vapor exiting the liquid outlet of theevaporator. At 2.8 hr, the evaporator heat load was shutoff—thisresulted in liquid exiting the vapor outlet of the evaporator. At 4.7hr, the condenser chiller was turned off—this eventually resulted invapor exiting at the liquid outlet of the evaporator. At 6.4 hr, thepump was turned off—this resulted in vapor being pushed out of all portsof the evaporator and the evaporator wick drying out. In between each ofthese perturbation events, the system was restored to a stable steadyoperating point by restoring the initial conditions. No hysteresiseffects or flow instabilities were observed.

Evaporator Performance: The SFA evaporator was able to produce separatedflow and verify the operation of the separated flow architecture.However, it was only possible to get the isothermal area to extendslightly beyond the heaters. FIG. 27 shows an IR image of the evaporatorwith 275 W applied near the outlet of the evaporator. The flow rate was79 mL/min. In this case, the temperature difference across the entireheat acquisition surface was 40° C. The cold inlet fluid can be clearlyseen at the inlet of the evaporator. The saturation temperature of thefluid was 72° C. This roughly corresponds to the whitish and red areason the image (excluding the rectangular heater) which indicates thepresence of vapor. While the vapor extends beyond the heaters somewhat,it clearly does not extend over the entire heat acquisition surface. Thesaturated vapor would need to extend over the entire heat acquisitionsurface, in order to meet the isothermality requirement.

This lack of isothermality stems from the fact that the liquid and vaporsides of the evaporator have a strong thermal coupling (as discussedabove). The evaporator is made of solid aluminum, and relies on a bulkybolted flange to attach the body and the cover plate. This means thatthe casing contributes to the strong thermal coupling of the liquid andvapor sides. Additionally, internal features of the evaporator alsocontribute to this coupling. Accordingly, embodiments may compriseconfiguration to thermally decouple the liquid and vapor sides of theevaporator, such as, for example, by using alternate designs andalternate (low conductivity) casing materials.

Summary

Based on preliminary findings, the SFA architecture is preferable to theMFA architecture on several grounds. Perhaps most importantly, SFAsystems are more predictable and amenable to analysis than MFA systems,since they predominately contain separated phases and single-phase flow.Additionally, for a given level of performance (power level,isothermality) an SFA system requires less power than an MFA systemsince no pre-heater is required for nominal operation. Finally, if nopre-heater is used, it appears that producing an isothermal evaporatoris more feasible with an SFA system.

An MFA system clearly has the potential to accommodate an evaporatorthat can meet the thermal requirements outlined above. However, thecosts associated with such a system are likely higher. Firstly, toachieve a high degree of isothermality will require using a pre-heaterto bring the working fluid up to saturation prior to the evaporator.While this required heater power can be offset by using a recuperatingheat exchanger between the evaporator inlet and outlet, there will stillbe a mass and/or power cost. The amount of pre-heat required could besignificant if the pump requires a high degree of subcooling. The secondsignificant issue with an MFA system is the lingering threat oftwo-phase flow instabilities and the attendant unpredictability oftwo-phase flow in microgravity. This introduces significant uncertaintyinto the design. A low power system (like the one required here) thatdoes not rely on using large pressure drops or high flow rates tosuppress instabilities is especially susceptible to these types ofinstabilities.

An SFA system circumvents the two major objections brought against MFAsystems. Firstly, it greatly reduces the risk of developing two-phaseflow instabilities by ensuring that the liquid and vapor phases remainseparated throughout the entire system (excepting in the condenser).This separation of phases also makes the system much more amenable toanalytic modelling. Secondly, an SFA evaporator has the potential tomeet the isothermality requirements described above without the need ofa pre-heater. It was experimentally shown that an SFA evaporator wasable to produce a small isothermal island at saturated conditions withfluid entering the evaporator 50° C. subcooled.

Doctrine of Equivalents

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof.Accordingly, the scope of the invention should be determined not by theembodiments illustrated, but by the appended claims and theirequivalents.

What is claimed is:
 1. A multi-phase evaporator comprising: anevaporator body having at least one outer heating surface and definingan internal volume; a vapor channel network comprising a plurality ofelongated heat transfer elements separated by a plurality of channels,wherein the elongated heat transfer elements have first endsinterconnected with an inner wall of the internal volume opposite atleast one outer heating surface such that the elongated heat transferelements and the outer heating surface are in thermal contact, andhaving second ends distal from inner wall; a wick element comprising aliquid porous body disposed in contact with the second ends of theplurality of elongated heat transfer elements; a fluid reservoirdisposed within the internal volume such that the wick element isinterposed between the fluid reservoir and the vapor channel network; atleast one fluid inlet disposed in a wall of the evaporator body suchthat fluid flows into the fluid reservoir; and at least one outletdisposed in a wall of the evaporator body distal from the at least onefluid inlet, the at least one outlet configured to transport both heatedvapor and heated liquid from the evaporator body.
 2. The multi-phaseevaporator of claim 1, further comprising: at least two outlets, whereinat least a first outlet is configured to transport heated liquid fromthe fluid reservoir, and wherein at least a second outlet is configuredto transport heated vapor from the vapor channel network; and whereinthe wick element is disposed within the evaporator body and configuredas a phase separator between the vapor channel network and the fluidreservoir.
 3. The multi-phase evaporator of claim 2, wherein the wickelement spans the entire opening between the fluid reservoir and thevapor channel network.
 4. The multi-phase evaporator of claim 1,comprising at least one mixed-phase outlet configured to transport bothheated vapor and heated liquid from the evaporator, and wherein the wickelement only partially spans the opening between the fluid reservoir andthe vapor channel network such that a mixing region is formed adjacentthe at least one mixed-phase outlet where heated vapor from the vaporchannel network and heated liquid from the fluid reservoir interactprior to flowing into the at least one mixed-phase outlet.
 5. Themulti-phase evaporator of claim 1, further comprising an evaporator capconfigured to seal the internal volume adjacent the fluid reservoir. 6.The multi-phase evaporator of claim 5, wherein the evaporator capfurther comprises a plurality of depressions formed into the innersurface thereof, the plurality of depressions being configured to engagea plurality of resilient members configured to engage and apply apreload force onto the wick element such that thermal contact ismaintained between the wick element and the second surfaces of theplurality of elongated heat transfer elements.
 7. The multi-phaseevaporator of claim 1, wherein the wick element is formed of a porousmaterial having variable porosity.
 8. The multi-phase evaporator ofclaim 7, wherein the porosity of the wick element varies continuouslythrough the cross-section of the wick element.
 9. The multi-phaseevaporator of claim 1, wherein the wick element is integrally formedinto the body of the evaporator.
 10. The multi-phase evaporator of claim1, wherein the wick element serves as a structural member of the body ofthe evaporator.
 11. The multi-phase evaporator of claim 1, wherein theat least one heating surface is configured to conform with the body ofan external heat generating element.
 12. The multi-phase evaporator ofclaim 1, further comprising a plurality of resilient members configuredto engage and apply a preload force onto the wick element such thatthermal contact is maintained between the wick element and the secondsurfaces of the plurality of elongated heat transfer elements.
 13. Themulti-phase evaporator of claim 1, wherein the plurality of elongatedheat transfer elements comprise pillars having a uniform geometriccross-section, the pillars being one of either solid or porous.
 14. Themulti-phase evaporator of claim 1, wherein the ratio of the width of alength determined by the width of an elongated heat transfer element andadjacent vapor channel to the width of the elongated heat transferelement is from 0.5 to 0.8.
 15. The multi-phase evaporator of claim 1,wherein the width of the elongated heat transfer element is from 0.25 to0.4 inches.
 16. The multi-phase evaporator of claim 1, wherein the wallsof the evaporator are formed from low thermally conducting materials.17. A thermal control system comprising: a fluid handling systemcomprising a plurality of interconnected fluid conduits; a pump in fluidcommunication with the fluid handling system; an evaporator in fluidcommunication with the fluid handling system and configured to pick up aheat load comprising: an evaporator body having at least one outerheating surface an defining an internal volume, a vapor channel networkcomprising a plurality of elongated heat transfer elements separated bya plurality of channels, wherein the elongated heat transfer elementshave first ends interconnected with an inner wall of the internal volumeopposite the at least one outer heating surface such that the elongatedheat transfer elements and the outer heating surface are in thermalcontact, and having second ends distal from inner wall, a wick elementcomprising a liquid porous body disposed in contact with the second endsof the plurality of elongated heat transfer elements, a fluid reservoirdisposed within the internal volume such that the wick element isinterposed between the fluid reservoir and the vapor channel network, atleast one fluid inlet disposed in a wall of the evaporator body suchthat fluid flows into the fluid reservoir, and at least one outletdisposed in a wall of the evaporator body distal from the at least onefluid inlet, the at least one outlet configured to transport both heatedvapor and heated liquid from the evaporator body; a condenser in fluidcommunication with the fluid handling system downstream of theevaporator and configured to reject the heat load; and an accumulator influid communication with the fluid handling system disposed at an inletof the pump and configured to regulate the system pressure.
 18. Thethermal control system of claim 17, further comprising: at least twooutlets, wherein at least a first outlet is configured to transportheated liquid from the fluid reservoir, and wherein at least a secondoutlet is configured to transport heated vapor from the vapor channelnetwork; and wherein the wick element is disposed within the evaporatorbody and configured as a phase separator between the vapor channelnetwork and the fluid reservoir.
 19. The thermal control system of claim17, comprising at least one mixed-phase outlet configured to transportboth heated vapor and heated liquid from the evaporator, and wherein thewick element only partially spans the opening between the fluidreservoir and the vapor channel network such that a mixing region isformed adjacent the at least one mixed-phase outlet where heated vaporfrom the vapor channel network and heated liquid from the fluidreservoir interact prior to flowing into the at least one mixed-phaseoutlet.
 20. The thermal control system of claim 19, further comprising apre-heater in fluid communication with the fluid handling systemupstream of the evaporator.